1. Field of the Invention
This invention relates to a video signal recording system, and more particularly to an automatic tracking control apparatus in a system for recording a video signal in successive parallel track sections on a moving recording medium.
2. Description of the Prior Art
When a signal having a broad band such as a video signal is recorded on, and reproduced from a magnetic recording medium, such as, a magnetic tape, the relative speed between the magnetic recording medium and a magnetic transducer or head for scanning the magnetic recording medium should be high. Generally, the video signal is recorded in successive parallel track sections on the magnetic tape, with such parallel track sections being in skewed relation to the longitudinal direction of the magnetic tape. In the usual video tape recorder (VRT), the video signal of one field or one frame is recorded in one track section on the magnetic tape. Each recorded track section is narrow so as to decrease the amount of magnetic tape used per track section and thus to enable longer play. Further, the guard bands between adjacent track sections are made narrow or eliminated for the purpose of high density recording. For such a VTR, various auto-tracking control techniques have been developed for ensuring that, during reproducing, the magnetic head is automatically moved to an optimum tracking position.
In an existing automatic tracking system used during play-back operation of a VTR of the helical scanning type, the reproducing magnetic head is deflected in a direction transverse to the length of the track for maintaining the optimum tracking position in slow or still reproduction. Such accurate tracking of the reproducing magnetic head avoids guard band noise which is produced from the guard bands between adjacent tracks. In such automatic tracking system, the reproducing magnetic head is fixed on one end of an electro-mechanical transducer element, such as a bi-morph leaf in which two piezo-ceramic plates, for example, of BaTiO.sub.3, PbTiO.sub.3 or PbNb.sub.2 O.sub.6 are laminated together. Control signals applied to the bi-morph leaf deflect the reproducing magnetic head in the transverse direction. The bi-morph leaf is light and strong, and its high speed response is excellent. However, it has some hysteresis and it becomes depolarized with age. Further, a damper for suppressing natural vibration of the bi-morph leaf assembly is normally arranged in contact with the bi-morph leaf plates. The damper may change its resilience with time. The circuits for energizing the electrodes of the bi-morph leaf assembly may also exhibit change with age.
For the above described reasons, it is difficult to accurately maintain the static home position of the bi-morph leaf assembly as the equipment ages. Thus, if recording operation is effected with the bi-morph leaf in its static home position and such position varies, the range of deflection of the bi-morph leaf may not be adequate during play-back or reproducing operation to achieve optimum tracking. A similar problem also exists in an auto-tracking apparatus of the moving-coil type in which the head position is controlled by a moving coil. Accordingly, in some VTRs, signals are recorded on the magnetic tape by means of a fixed magnetic head, and reproduction of such signals from the magnetic tape is effected by another magnetic head supported on the bi-morph leaf assembly.
When separate magnetic heads are used for recording and reproducing signals, respectively, with the recording head being fixed and the reproducing head being movable, as aforesaid, the picture is frequently deteriorated due to the difference in quality of the recording and reproducing magnetic heads. The requirement to compensate for this difference in quality results in increased cost. In addition, since the permissible range of the deflecting movement of the reproducing head is limited, such movement may not be sufficient to permit accurate tracking. Reproducing a tape on one VTR which was recorded on another VTR increases the tracking problem even further.